There is a need for a simple and quick way to align any two optical instruments so that their optical axes are exactly parallel. The need is particularly acute in the case of missiles such as TOW and HELLFIRE whose successful impact on the target depends on the axes of their guidance lasers, beam projectors or optical trackers being pointed with extreme accuracy at the target. Such a guidance device usually incorporates a day-sight (direct-view optical telescope) which is boresighted (brought into parallelism) to within 10-50 microradians of the optical axis of the guidance device. Alignment of such a high degree of accuracy cannot be easily set or adjusted in the field.
Where a tactical application precludes field adjustment, it is usually sufficient to use a guidance system that is designed for permanent internal alignment, in the factory, of the optical axes of the guidance device and the day-sight. However, permanent factory alignment is impractical when field installation is necessary, as when a night-sight attachment is needed. Mechanical attaching means, including kinematic mounts, are incapable of providing repeatably the required degree of accuracy in the optical alignment of the axes of the guidance device and the attached night-sight. At present, a night-sight is mechanically attached by the soldier to the guidance device at the point of use. Since this action usually leaves the two units firmly joined but misaligned, the current practice is to attach a boresight collimator, which unites the two objective optical axes within the collimator instrument, thereby allowing precise alignment. The disadvantage of this approach is that the collimator and its case, weighing at least ten pounds, must be carried on every mission that may require a night-sight. Further, the joining and aligning operation must be repeated every time even a gentle shock is applied to the assembly, such as moving a short distance or replacing the battery. In addition, the boresight collimator itself is a precision optical instrument that must be maintained in its internal alignment by means of a boresight collimator test set.
The interferometric alignment device is a small, compact device that can easily be attached to and detached from any two optical instruments, such as a laser designator and its night-sight, that need to be aligned precisely in pitch and yaw angles. The alignment device utilizes light reflecting from mirrors that are permanently mounted inside the instruments to be aligned, one mirror in each instrument. The light beams reflecting from the internal mirrors exit their respective instruments via a window built into the frame of the instrument and re-enter the alignment device wherein they combine to form an interference pattern. The operator of the alignment device observes the fringes of this pattern and adjusts the azimuth and elevation of one instrument relative to the other instrument until the fringes are at a minimum in number or are eliminated altogether, thereby leaving the field-of-view uniformly illuminated.